1. Field of the Invention
The invention relates to glycoprotein synthesis, and more particularly, to the synthesis of homogeneous glycoproteins wherein one or more oligosaccharide sugar chains with a predetermined number of sugar moieties are added to a GlcNAc-containing polypeptide/protein and wherein the oligosaccharide sugar chains may further include a functional moiety.
2. Description of the Related Art
Glycoproteins are an important class of biomolecules that play crucial roles in many biological events such as cell adhesion, tumor metastasis, pathogen infection, and immune response [1]. However, a major problem in structural and functional studies of glycoproteins is their structural micro-heterogeneity. Natural and recombinant glycoproteins are typically produced as a mixture of glycoforms that differ only in the structure of the pendent oligosaccharides.
Most mammalian cell surface proteins and human serum proteins are glycoproteins and therapeutic glycoproteins are an important class of biotechnology products. They include granulocyte macrophage-colony stimulating factor, tissue plasminogen activator, interleukin-2, and erythropoietin (EPO), which alone generates sales of 3-5 billion dollars annually.
A major challenge in preparation of protein-based drugs is post-translational modifications (glycosylation, phosphorylation, acetylation, etc.) that cannot be deduced directly from the sequence. Elucidating the roles of post-translational modifications has a direct impact on development of therapeutic glycoprotein. Glycosylation is one of the most common post-translational modifications of proteins in eukaryotes. Control of proper glycosylation is of major importance in the development of glycoprotein and glycopeptide drugs, because the attached sugar chains can have profound effects on protein folding, stability, action, pharmacokinetics, and serum half-life.
Therapeutic glycoproteins are typically produced in cell culture systems as a mixture of glycoforms that possess the same peptide backbone but differ in both the nature and site of glycosylation. The heterogeneity in glycosylation poses significant difficulty in the development of glycoprotein drugs. For example, expression of EPO in E. coli has been found to result in non-glycosylated EPO that shows only minimal activity, and EPO overproduced in plant cells such as tobacco cells has been found to produce no biological activity in vivo, presumably due to a high clearance rate resulting from a lack of masking sialic acid residues in the N-glycans.
Cell engineering and some biochemical modifications have yielded recombinant glycoproteins with predominant glycoforms but, in most cases, as with natively expressed glycoproteins, the structures that have been obtained remain heterogeneous. Notably, some glycoforms can cause allergy problems and undesired immune responses.
Antibodies, especially monoclonal antibodies (mAbs) are emerging as an important class of therapeutic agents for the treatment of human diseases such as cancer. Currently, antibodies used for treatment are the IgG type and are produced in mammalian cells (CHO cells or mouse NSO cell lines etc.). All types of antibodies including monoclonal antibodies are glycoproteins. More and more evidence have shown that different glycosylation forms can exert significantly different effects on the properties of a given therapeutic antibody, some sugar chains are beneficial, while others are detrimental [1d, 1e]. Unfortunately, recombinant mAbs are usually produced as a mixture of various glycosylation states, in which the more active glycosylation states (e.g., de-fucosylated and/or bisecting GlcNAc-containing N-glycans) that demonstrate enhanced ADCC effector functions are present only in minor amounts [1e].
A typical immunoglobulin G (IgG) antibody is composed of two light and two heavy chains that are associated with each other to form three major domains connected through a flexible hinge region: the two identical antigen-binding (Fab) regions and the constant (Fc) region. The IgG-Fc region is a homodimer in which the two CH3 domains are paired through non-covalent interactions. The two CH2 domains are not paired but each has a conserved N-glycosylation site at Asn-297. After the antibody's recognition and binding to a target cell, ADCC and other effector functions are triggered through the binding of the antibody's Fc region to ligands or receptors.
It is noted that there are heterogeneous glycosylation states of the human IgG when expressed in mammalian cell lines (e.g., CHO cell lines), and isolation of human IgG having a particular glycosylation state from this mixture is extremely difficult. Small amounts of impurities of a highly active species can dramatically interfere with the results and data interpretation.
As such, there is a need for proper and consistent glycosylation in developing glycoprotein therapeutic agents including antibodies to reduce allergy problems or undesired immune responses; confer significant stability and effector activity of an antibody and for compliance with US FDA regulations to obtain market approval.